The present invention relates to a rotary actuator, especially to hydraulic rotary actuators used to rotate aileron and other flaps on airborne frames. Vehicles moving through air rotate, extend and retract control surfaces to deflect the air so the vehicle rotates in response to the force on the control surface or changes speed in response to the forces on the control surfaces. Control surfaces on wings and tails of airplanes are commonly recognized examples. If the rotary actuators are sufficiently small, they may be placed along the rotations axis of the control surface. But such in-line actuators often lack the torque capacity required to rotate the control surfaces. Further, to achieve the required torque capacity, the rotary actuators become larger and heavier than desired, requiring them to be mounted off-axis and use intervening mechanisms to connect to the control surface. For example, a gear mechanism or linkage mechanism may be used to increase the applied force and connect the rotary actuator to the control surface, but the mechanism increases cost and complexity, delays the response time and reduces the stiffness of the drive mechanism for the control-surface. Additionally, linear actuator assemblies may be used having a linear actuator with an intervening mechanism to vary the power and/or convert linear motion into rotary motion, but such assemblies suffer from the same and additional disadvantages as the gear and linkage mechanisms. There is thus a need for an improved rotary actuator.
There is also a need for a rotary actuator that eliminates cross chamber leakage, is inexpensive, has high torque density, and is rugged. Cross chamber leakage adds to operating cost as the energy used to pressurize hydraulic fluid is constantly dissipated so that it and position control must always be active for a conventional vane actuator to maintain a desired position. Conventional rotary piston actuators may accomplish position hold without constant active control, but their construction is inefficient as far as cost, part count, size and weight. Size and weight may be primary factors for use in many applications. Size and weight are especially important in flight control applications where thin wings are desired for low drag in wing design.
Even before the beginning of jet flight it was recognized that thin wings enable faster velocities. One component that has been considered desirable is a powered hinge, or rotary hydraulic actuator, to eliminate the bell crank that is necessary to convert the linear motion of a conventional actuator to the rotary motion of a flight control surface about its hinge line. Several concepts have been developed for the powered hinge type. Among them are the vane actuator, the geared actuator, the radial piston actuator, and the rotary piston actor.
But the vane actuator can achieve a high torque density and range of motion but lacks a sealing method to prevent cross chamber leakage that would enable hydraulic blocking to hold control surface position without constant adjustment and are sluggish in their response to commands. The seals for vane actuators have proven unreliable under the harsh demands of flight control which causes the actuators to suffer a very short lifespan before they need to be overhauled and the seals replaced. The geared actuator can be packed relatively thin in profile and with a high torque density, but by sacrificing of command response and with a high degree of complexity in their gear train. The gear train also has an inherently low rate of reliability that is intolerable in a flight control actuator. Radial piston actuators have a relatively low torque density, which makes them poor candidates for flight control actuators for a thin wing design. There is thus a continued need for an improved rotary actuator.
Rotary piston actuators have been in use for decades but not considered for a flight control until recently. The conventional design uses a center output shaft that consist of a lever that extends radially outward to connect with curved pistons that reciprocate into and out of similarly curved cavities of a housing through conventional piston seals. This arrangement is enclosed inside an enclosure to protect it from atmosphere and foreign debris, while also protecting the surrounding atmosphere from leakage that inherently occurs as the pistons reciprocate through the cavity seal. The rotary piston actuator suffer in torque density as each components' size increases the overall size, especially if it is desired to achieve a high torque output and stiffness.